For one, the Queen – who always seems completely self-reliant in public – actually leans heavily on her husband Prince Philip for moral support, according to Harry.

"Regardless of whether my grandfather seems to be doing his own thing, sort of wandering off like a fish down the river, the fact that he's there – personally, I don't think that she could do it without him," says Harry, who was interviewed with his brother by the BBC's Andrew Marr for an upcoming documentary on the Queen. Harry and William's quotes have been included in a piece in the Radio Times out in the U.K. on Tuesday.

The prince, 27 – who was interviewed before Philip underwent emergency surgery for a blocked coronary artery just before Christmas – says his grandmother would find the task of endlessly shaking hands during public engagements almost unbearable without her husband by her side.

Harry worries that his grandmother's rigorous schedule of opening hospitals, meeting politicians and greeting members of the public could be too taxing for the 85-year-old royal, who celebrates 60 years on the throne next Monday. "Yet she's carrying on and doing [the visits]," says Harry.

William is very close to his grandmother and is said, by those close to him, to take his lead from her. Both he and wife Kate look to the Queen for guidance about how to lead a public royal life.

"I think she doesn't care for celebrity ... and she really minds about having privacy in general," William tells Marr.

The prince also praises how his grandmother navigates "a very tricky line" between "private and public and duty."

"I think she's carved her own way completely. She's not had a blueprint," says William. "I think it's very important to be able to retreat inside and be able to collect one's thoughts and collect your ideas, and then to move forward."